C program to simule a simple calculator

Question

I am learning C program. I wrote a program to create a calculator using switch statement. When I tried to run this code the program stops in the middle of it when running, I realized what the problem was, I show you my three attemps, the first one is the correct one but I woud like to know why the others two didn't work.

  // Calculator
    #include <stdio.h> 
   int main ()
   {
        int num1, num2;
        char key;
        printf("CALCULATOR \n");
        printf("choose +, -, *, / or %% \n");
        scanf("%c", &key);
        printf("Enter two integers\n");
        scanf("%d %d", &num1, &num2);

        switch(key)
              {
                   case '+': printf("%d", num1 + num2);
                           break;
                   case '-': printf("%d", num1 - num2);
                           break;
                   case '*': printf("%d", num1 * num2);
                           break;
                   case '/': printf("%d", num1/num2);
                           break;
                   case '%': printf("%d", num1%num2);
                            break;
                   default: printf("syntax error");                 
              }
                  return 0;
     } // the correc one 

but these two are wrong, I want to know why if the logic is the same.

            // Calculator
            #include <stdio.h>
            int main ()
            {
                int num1, num2;
                char key;
                printf("CALCULATOR \n");
                printf("Enter two integers\n");
                scanf("%d %d", &num1, &num2);
                printf("choose +, -, *, / or %% \n");
                scanf("%c", &key);
                
                switch(key)
                {
                    case '+': printf("%d", num1 + num2);
                        break;
                    case '-': printf("%d", num1 - num2);
                        break;
                    case '*': printf("%d", num1 * num2);
                        break;
                    case '/': printf("%d", num1/num2);
                        break;
                    case '%': printf("%d", num1%num2);
                        break;
                    default: printf("syntax error");                    
                }
                return 0;
            } //wrong

the last one

            // Calculator
            #include <stdio.h>
            int main ()
            {
                int num1, num2;
                char key;
                printf("CALCULATOR \n");
                printf("Enter a integer\n");
                scanf("%d", &num1);
                printf("choose +, -, *, / or %% \n");
                scanf("%c", &key);
                printf("Enter a integer\n");
                scanf("%d", &num2);
                
                switch(key)
                {
                    case '+': printf("%d", num1 + num2);
                        break;
                    case '-': printf("%d", num1 - num2);
                        break;
                    case '*': printf("%d", num1 * num2);
                        break;
                    case '/': printf("%d", num1/num2);
                        break;
                    case '%': printf("%d", num1%num2);
                        break;
                    default: printf("syntax error");                    
                }
                return 0;
            }

I'd appreciate an answer.